The invention relates, generally, to surveying systems and, more particularly, to an improved prism pole for such systems.
Surveying systems are used for mapping and plotting purposes by identifying the location of any point on a three dimensional coordinate system as graphically illustrated in FIG. 1. The typical surveying system consists of a prism pole 1 supporting a prism assembly 5 for positioning the prism over the point P to be located, and an electronic distance measuring device 3 for measuring to the point and automatically assigning three dimensional coordinates. These coordinates can then be transferred to a computer for design or mapping purposes.
Prism poles constructed of a high strength aluminum, typically consist of an outer-bottom section and at least one inner telescoping section. The outer section having a hardened steel point on the bottom end thereof to permit its being precisely positioned on the point being measured. A prism assembly, which is mounted on the top end of the pole, serves as the target for the infrared beam generated by the electronic measuring device, reflects the beam back to the measuring device.
In order to ensure accurate measurements, the prism pole must be held plumb with the prism assembly aligned directly above the point being measured. Because the typical prism pole is at least 56 inches long when fully collapsed, it is difficult to keep the prism pole plumb and therefore the prism assembly over the point being measured. To assist in plumbing the prism pole, most poles are provided with a level vial observable by the operator while positioning the prism pole. Unfortunately, these level vials are only accurate to .+-.45 minutes of arc. Any error in the vertical alignment of the prism pole will result in measurement errors. Moreover, the longer the prism pole, the greater the measurement errors will be as illustrated in FIG. 2, where the error in vertical alignment is shown by angle .alpha..
Existing prism poles are available in a variety of sizes that typically extend to lengths of between 4 and 151/2 feet. Longer prism poles are needed in areas where hills or other obstacles would otherwise obstruct a short prism pole. The applicant has recognized that there are applications where relatively flat, unobstructed surfaces are measured, for example, the inside of buildings or paved surfaces. For these applications, long prism poles are not required and their use increases the risk that the pole will be misaligned, thereby introducing unnecessary errors. For these applications, a more easily positioned pole that minimizes the error due to misalignment is desired.